I suspected Lightning because I noticed my posts on the MozillaZine forums only showed Lightning 1.04bpre as my UA instead of the full build string.

I just installed the latest nightly and the results are the same. Also created a test profile without Lightning and Yahoo! Mail Beta worked. Installed Lightning 1.0b4pre and Yahoo! Mail Beta was disabled again.

Just to be sure I even downloaded and re-installed SM 2.1 RC1. Lightning is disabled on this post.

I suspected Lightning because I noticed my posts on the MozillaZine forums only showed Lightning 1.04bpre as my UA instead of the full build string.

I just installed the latest nightly and the results are the same. Also created a test profile without Lightning and Yahoo! Mail Beta worked. Installed Lightning 1.0b4pre and Yahoo! Mail Beta was disabled again.

Just to be sure I even downloaded and re-installed SM 2.1 RC1. Lightning is disabled on this post.